1. Field of the Invention
The present invention relates to a receiving method and a receiver for compensating a received signal with a timing offset and a chromatic dispersion obtained by estimation in a system adopting an optical orthogonal frequency-division multiplexing (OFDM) technique.
2. Related Art
An OFDM system is a frequency-division multiplexing (FDM) system adopting a digital multi-carrier modulation method, in which a plurality of orthogonal sub-carriers (or called sub-bands) is used for data transmission. The data is divided into a plurality of parallel data streams or channels corresponding to each sub-carrier respectively. Each sub-carrier transmits the data through a modulation technique with a relatively low symbol rate (for example, quadrature amplitude modulation (QAM) or phase shift keying (PSK)). In this manner, a larger total data rate is achieved in the same bandwidth as compared with that of the single-carrier in the conventional art.
FIGS. 1A and 1B are respectively schematic comparative views of a frequency spectrum distribution of a conventional direct transmission and an OFDM transmission. Referring to FIGS. 1A and 1B, the most significant difference between the direct transmission and the OFDM transmission lies in the bandwidth distribution. Referring to FIG. 1A, the bandwidth occupied by the direct transmission is f0. If the bandwidth f0 is equally divided into five parts f1 with the same width in an OFDM manner, and the sub-bands f1 (that is, the above sub-carriers) are orthogonal with one another, so that the new frequency spectrum distribution is shown in FIG. 1B. In the OFDM transmission, as long as the sub-bands are enough, for each sub-band, basically the frequency response of the sub-band may be approximately considered as flat. That is to say, each sub-band only requires one equalizer with a single coefficient, so as to overcome the attenuation and phase distortion of each sub-channel. In addition, the data rate transmitted by each sub-band is much lower than that of the conventional direct transmission, and an operation clock of the equalizer is naturally reduced proportionally.
When being applied to the wireless communication field, the OFDM technique often faces a common problem of a multi-path effect. The multi-path effect may result in the time-spreading and inter-symbol interference (ISI) problem, which is the so-called frequency-selective channel problem. Usually, the frequency-selective problem is solved by adding a guard interval to each symbol of the OFDM, such that the symbol period is increased accordingly, which further occupies the bandwidth used for data transmission.
When the OFDM technique is applied to an optical communication system, light rays are transmitted in the same optical fiber, so the multi-path effect of the optical OFDM system is not significant. But, due to the chromatic dispersion phenomenon, when a receiving end receives a signal, the synchronization problem among channels and similar multi-path ISI problem exist. The synchronization estimation manner in the optical OFDM system is somewhat different from the conventional estimation manner on the copper wire transmission or wireless OFDM transmission. Therefore, it is necessary to develop a new synchronization estimation method for the optical fiber channel characteristics, thereby avoiding the estimation faults.
In the OFDM system, the techniques relevant to the synchronization estimation (that is, estimation of the timing offset) performed by the receiving end are provided in U.S. Pat. No. 7,310,302, filed on Dec. 18, 2007 and entitled “Method for estimation time and frequency offset in an OFDM system” and U.S. Pat. No. 7,251,283, filed on Jul. 31, 2007 and entitled “Timing offset compensation in orthogonal frequency division multiplexing systems”.
In addition, the relevant papers include papers issued by Minjian Zhao, Aiping Huang, Zhaoyang Zhang, and Peiliang Qiu (with reference to All Digital Tracking Loop for OFDM Symbol Timing, IEEE VTC'2003, pp. 2435-2439, vol. 4, October 2003), and papers issued by Baoguo Yang, Khaled Ben Letaief, Roger S. Cheng, and Zhigang Cao (with reference to Timing Recovery for OFDM transmission, IEEE J. Select. Areas Commun., Vol. 18, No. 11, November 2003).
In the conventional art, the method for estimating timing offset applicable to the wireless or copper wire OFDM system has been proposed, but the characteristics of the optical fiber channel are not entirely the same as that of the wireless or copper wire transmission, so that the synchronization estimation manner in the optical OFDM system is different from the conventional estimation manner. The accuracy of the estimation on a symbol boundary may affect the performance of the OFDM frequency domain equalizer or channel estimation, so as to affect the accuracy for demodulating the transferred signal. That is to say, the synchronization estimation and channel estimation techniques directly affect the performance of the entire OFDM system. Therefore, a new synchronization estimation method directed to the optical fiber channel characteristics is required, thereby preventing the estimation faults.